Crystalline silicon carbide exists in many chemically different forms such as the hexagonal crystal form .alpha.-SiC, particularly 6H-SiC, and the cubic crystal form .beta.-SiC (3C-SiC).
SiC semiconductor devices are of great interest in view of the fact that the thermal conductivity is 330% higher in SiC than in Si and 10 times higher than in GaAs, that the breakdown electric field is 10 times higher in SiC than in both Si and GaAs, that the saturated electron drift velocity which is the speed limit for electrons in a material, is 40-50% higher in SiC than in both Si and GaAs, and that the wide bandgap in SiC enables operation at temperatures higher than 500.degree. C.
Most semiconductor devices need terminal connections to carry electric current to and from the internal of the semiconductor device. Such a terminal connection, usually called an ohmic contact, must not however impair the semiconductor device itself. Thus, the voltage drop over the ohmic contact should be negligible compared to the voltage drop across other areas of the semiconductor device at the current density in question.
From the article "High Frequency Performance of SiC Heterojunction Bipolar Transistors", G. Guang-bo et al., IEEE Transaction on Electron Devices, Vol. 41, No. 7, Jul. 1994, pp. 1092, it is apparent that a specific contact resistance of less than 0.0001 .OMEGA..cm.sup.2 for p-type SiC, is required if silicon carbide devices are to have a future in comparison with Si and GaAs devices.
Aluminium has been considered a potential contact metal but its low melting point, 660.degree. C., makes it less ideal at high power or high temperature operation. Another problem with aluminium is its reactivity with oxygen that may result in isolating oxides.
In "Annealing Effects on Al and Al-Si Contacts with 3C-SiC", H. Daimon et al., Japanese Journal of Applied Physics, Vol. 25, No. 7, Jul., 1986, pp.L592-L594, the Al-Si contact electrodes are described after heat treatment from 200.degree. C. to 1000.degree. C. The Al-Si contact was considered as an ohmic contact but the resulting electrode was found to be inhomogeneously granular with potential reliability problems.
Another attempt to obtain a high performance ohmic contact on 3C-SiC is described in U.S. Pat. No. 4,990,994 and U.S. Pat. No. 5,124,779 where silicon is replaced by titanium in combination with aluminum. Heat treatment is preferably avoided due to the above mentioned uniformity problems. A linear current-voltage characteristic for n-type 3C-SiC material is presented.
JP 4-85972 relates to a method of reducing contact resistance of a p-type .alpha.-silicon carbide with an electrode, and making the ohmic properties in the electrode uniform by laminating an Ni film, a Ti film in an arbitrary sequence on the SiC, laminating an Al film thereon, and then heat-treating it. The specific contact resistance is not sufficiently low for a high performance ohmic contact, and nothing is stated about the stability properties at elevated operating temperatures.